Transmission



May 28, 1940. c. w. HANSELL TRANSMISSION Filed June 5, 1936 3 Sheets-Sheet 1 his fiaqwu lNVENTQR CLARE EW.HAN5ELL BY ATTORN EY May 28, 1940. c; w. HANSELL TRANSMISSION Filed June 5', 1936 3 Sheets-Sheet 2 IVVVVVVVVVVVVVVVI+ AAAAAAAAAAAAAAAAA R w M m M m MODUZA r50 ourPur INVENTOR CLARENCE w.HANsELL BY g 1 CARR/ER ATTORNEY INPUT May 28, 1940,

c. w. HANSELL TRANSMISSION Filed June 5, 1936 3 Sheets-Sheet 3 I! c C R 2 x .m M w 7 T MT 0 5 U M R R h AN 6 C l 6 YR .A E R T A 5 M m m m I F FREOUENC Y I m ma May 2 8, 1940 a i STATES PAT NT orrlca mssnnssron Clarence W. Bansell, Port Jefferson, N. Y., as-

signor to Radio Corporation of America, a I I corporation of Delaware 1 Application June 5, 1930, Serial No. 83,643 1 Claims. (c1. its-7.1)

.10. made to the use of the same in facsimile workv although I contemplate the use of my method and means in allknown types of signalling systems, including television in particular, since television is little more than a high speed facl-ii simile transmission system in which no attempt is made to produce a permanent record of the received pictures.

One of the defects present in known picture and facsimile transmission systems is the loss of definition occurring in the transmitter circuits, during transmission, and in the receiver circuits or due to the same and to poor definition of the message being scanned, which becomes apparent in the reproduced'message. 25 simile transmission of typewritten material this may show up due to blurred letters or parts of letters or change in density of background which is accentuated by imperfect transmission and which causes loss of definition and intelligibility 30 in the received message. An object of the presentinvention is to provide a facsimile or television transmission circuit including novel means for correcting this loss of definition inherent in present systems and for increasing or exagger- 5 ating the definition present in the original message so that the reproduced copy may be equal to or even more intelligible than the original. Other objects will become apparent from the'description of the method and means for accomplishing the main object. I

In one form of my inventionI supplement the direct currentpotential changes, -or variations in current intensity, produced by scanning the message, by additional derived potentials which 45 may 'vary substantially in proportion to the rate of change of light produced by scanning the message. Both variations are then caused to energize amplifying or relaying means and good definition of the mesage is insured more nearly 60 irrespective of the condition of the original message or loss of definition in the transmission circuits and medium. The potentials I produce, which vary substantially in proportion to the rate of change of light produced byv scanning,

d8 canbe made to increase the percentage variation In facin potential or current relative to the percentage variation in the light produced by scanning. In other words, the changes or rate of changes in 1 light intensity, .as compared to the average or total intensity, are made to have a greater or a exaggerated eil'ect upon the scanner output current. In this manner loss of definition due to any cause is reduced.

In some cases entire control is desirable by means responsive to the rate of change in light. 1 caused by scanning. For example, rate of change control is particularly applicable to typewritten and similar material where the periods of scanning black are short compared with the periods of scanning white. Rate of change control completely eliminates the problem of adjustment for background in the transmission of plain messages. In handling commercial messages the direct current control may be cut in only when the message has large black areas too wide to be handled by the rate of change control.

In describing my invention reference will be made to the attached drawings throughout which insofar as possible, like characters indicate like parts and in which,

Figures 1 and la illustrate a transmission system by means of which a wide band of signal currents or potentials of possibly poor definition are caused to produce a correspondingband of currents or potentials modified in such a manner that they represent corresponding signals of good definition and means for placing the same on a carrier wave for transmission purposes;

Figures 1b, 2, 3, and 4 show mod'flcations of the arrangement of Figure 1. In each of Figures 2,

3, and 4 I, have shown different circuit arrangements for producing the controlling currents or potentials which arepartially or fully responsive more or less in proportion to the rate of change of light produced by scanning and means ener- 4 gized by said currents or potentials alone or by the same with currents the intensity of which are characteristic of the signals for controlling wave energy;' while Figure 5 is a curve. illustrating the characteristics'of the rate of change potential producing circuits.

. Referring to Figures 1 and 1a of the drawings, themessage to be scanned maybe carried by a drum III cooperating with a source of light 5i) l2 and a lens system [4, i6, and diaphragm l8 to produce a ray of light falling on the input electrode 20 of a tube 24 the output electrode 26 of which is connected by way of a resistance 28 to a source of potential shown for convenience as 2 I I a'aoaeaoj a potentiometer resistance PR. The potential variations produced across 23 are applied to the screen grid electrode 30 of an electron-discharge device 32 the anode of which is connected with the plate circuit of a full wave modulator l0.

The fullwave modulator 40 has its input electrodes coupled by transformer 42 toa source of carrier wave energy.

Full wave modulator 40 has itsv anode elecplier amplifier constituted by a plurality of pairs of opposed plates axially disposed in an evacuated containerwith their faces lying in parallel planes as shown in Figures 1 and 1a. For convenience the upper electrodes or plates have been designated in the art .by the term "upper or "accelerating electrodes" and the lower plates have been designated bythegterm "lower or multiplying electrodes." of the multiplying electrodes arespecially prepared toenhance their ability to emit secondary electrons when they are struck 'by electrons arriving from other electrodes. Variations in light intensity falling upon electrode causes variations in the number of electrons emitted from it. These electrons under the influence of electrio and magnetic fields present in the tube are caused to strike the next adjacent secondary emission electrode, producing secondary electrons the number of which is multiplied in the tube. The electron device 24, when so energized, produces a greatly amplified output in the circuit 28, 23. An electro-magnetic means in the form of winding 23, on a magnet2'l, energized through variable resistance 25 by a source 23, is provided for the purpose of causing the electrons emitted from 20 under the action of the light ray and from the succeeding lower plates to describe approximately trochoidal paths from each lower electrode to the next adjacent lower electrode, thus permitting the use of high accelerating potentials on the upper electrodes.

The secondary emission produced by electronsstriking the plates increases the current arriving at successive electrodes until finally the current arriving at anode 26 may be several million times the photo current from electrode 20, if the tube has a sumcient number of stages.

This'brief description of this electron multiplying tube is sumcient for the purposes of the present invention. For those interested, a fuller description of said tube may be found in United States patent to V. K. Zworykin, 2,078,304, issued on April 27 193'7.

As pointed out briefly above, the screen grid 30 of tube 32 is energized by the amplifier potentials in the output circuit 26 to control the amplifier 32 which in turn modulates the modulator 40. The screen grid potential is made to vary in response to the total light falling on the electrode 20. By means of this control the light when varied from maximum to minimum value may make the audio output from the modulated tubes 40 change from zero to maximum.

Preferably the faces I In my novel method and means I also produce rate of change control potential and impress the same on the control grid 3| of tube by variable resistor Rand variable capacity C connected as shown'between a point on 23 and the control grid of tube 32. This resistive-reactive coupling R, 0 causes the control grid potential to vary more or less in proportion to the rate of change of screen grid potential. As a result, the natural loss of definition due to 'variable reflecting power of the paper, variable color of paper, poor typing, size of the scanning spot, etc., is reduced.' If the response to the rate of change of light falling on the photocell is made I greatenough all variations in the light produce a still greater percentage of variation in the output from the modulatortubes. 1

g This increase in percentageresponse tends to overcome the unavoidable'loss of definition in scanning thesubject. The response to rate of change of control grid potential may be controlled by adjusting the resistance R and the reactance C. Normally the response of R, 0 increases with increase in frequency, or rate of change of the current, in 23v up to rates of change at which the reactance of C is a negligible factor in the circuit. A more flexible 'control of the rate of change potentials may be? had by supplementing the action of R, C by a capacity 01 shunting the resistance 33 as shown in Figure 1b. If the values of R, C and Cl are properly adjusted the amplitudes of the rate of change potentials supplied by R, C to the con- .trolling electrode grows as the frequency of the initiating current increases until a preselected upper value is reached at which frequenciesthe condenser 01 of Figure 1b shunts the potentials so that they are ,of decreasing value above this frequency. The characteristic of the circuit R,

1 s2. additional potential may be produced C, 33 and of the circuit R, C, C1, 33 of Figure 1b, taken in combination with the other circuit elements, has been illustrated in Figure 5. ,These response characteristics of Figure 5 will, oi"

course, be further modified, by limitation in rate of change determined'by thesize of the scanning spot, by filters and other elements associated with the system. I

In general R and C and C1 are so adjusted that the rate of change potentials produced by variations in current intensity are proportional to the value of the desired frequencies, and the desired frequencies are determined by taking into account not only those frequencies required to reproduce the message accurately but also those frequencies which may be permitted on the circuit without interfering with other channels or circuits.

In practice R and C or R and C and C1 are adjusted to produce optimum permissible signal definition. The other electrodes of the tube are energized as shown and a description of the .energizing circuits for these electrodes is thought to be unnecessary.

Biasing potentials for the control grid of 32 are supplied through resistance 33 connected as shown to any source of potentials. Any alternating current appearing in resistance 33 may 'be shunted out of the power source by a condenser 34.

For. scanning typewritten material or other subjects where the periods of scanning black are short compared with the periods of scanning white, or vice versa, it is possible to employ nothing but rate of change response, thus eliminating the direct current element of control altogether. This eliminates the problem of adjustment for background and transmission of plain messages. The direct current control may be cut in only when the object has large black areas too wide to be handled by the rate of change control. The switching means S in Figure 1 permits the use of the reactive circuit RC along or supplemented by the direct current control on 30.

There are, of course, a great number of detail circuits by means of which rate of change modulation or keying may be applied to an electrode in the amplifier tube 32. In general, they all involve a capacity or a transformer coupling of some kind for transferring alternatingv potentials or current. When the alternating current component of coupling is made large in comparison with a parallel direct current coupling,'or when the alternating current component is greater than the threshold and limiting values passed on by a a succeeding limiter amplifier not shown in the drawings, then an increase in definition or an exaggeration of variations in input currentis obtained.

In Figure 2 I have shown a transformer means 50 for stepping up the rate of change potentials and applying the rate of change modulating .po-

tentials to the control grid 3|. The rate of change modulating potentials supplement or replace,"ac-

cording to the position of switch S, the modulating current potential applied to the screen electrode 30.

In Figure 3 the thermionic tube 32 is shown as a 'triode having electrodes including. a control grid 3| and the direct current potentials characteristic of light .intensity as well as the potentialswhich vary in accordance with rate of change of light are applied to the control electrode 3| by way of an auto-transformer 52. The primary of this transformer is connected with the output impedance 28 as shown by way of a condenser C4 large enough to pass the alternating .current component of. the output of 2 4. The alternating current componentpotentials are stepped up and appear in the secondary of from which they are supplied to the grid of 32. Figures. 2 and 3 are in otherrespects suificiently similar to the circuit of Figure 1 to eliminate'the necessity of describing these figures in detail.

In the modification-of Figure 4 the rate of change modulating potentials are supplied by way of a variable reactive element 60 from the high permits the rate of change potentials to be regulated from minimum to maximum. These adpotential end of resistor 28' whichalso .supplies the currents the intensity of which are characteristic of light intensity directly to the screen electrodes of the modulatortubes in 40 which are in this embodiment of the screen grid type. Movement of the point 64 on 28' permits the direct current to be regulated from minimum to maximum, while adjustment of the value of i justable controls permit control of the modulator byeither or both the currents of varying intensity or the potentials which are of ampli- M t'ude commensurate withthe rate'ofchange of the intensity of the varying currents.

The carrier wave potentials are supplied to the N control grids of the tubes of modulator "as in Figures 1, 2, 3, while the modulated wave energy is taken from the anodes as in the prior figures.

' Employment of rate of change control at the transmitter. is of, course, chiefl'yfeffective in reducing loss of],definition in" the transmitter end.

If there is further loss of definition in transmission, including that due to filters, tuned circuits, etc., then similar circuits may be used at the receiver for increasing the definition again. Also, if desired, all increase of definition may be made at the receiver, though this may involve som loss in eifective signal-noise ratio.

It may be noted that rate of change tends to give the same result as would be obtained if the length of spot covered by the'transmitter scanner were reduced and this combined with limiting at top and bottom of the signal character can make the system more independent of density of the marks on the paper and of the density of the background formed by the paper. This enhancement of definition may be carried to a point to which manual adjustment of the scanning and transmitting apparatus by the operators when changing messages having widely different background density and color and widely varying marking density and color will be dispensed with. Although I have shown a tube of the electron multiplier type for converting the scanned light ing with a discharge device in a circuit of appropriate design to convert the scanned message into controlling currents and potentials.

I have illustrated my invention with figures most appropriate to facsimile transmission in which the object to be scanned is moved to accomplish the scanning but it will be obvious to anyone skilled in the art that my invention applies equally well to television scanning by moving electron beams as describedv by Zworykin and Farnsworth. Application of my invention to both transmission and reception of television images -will result in marked improvement in definition,

particularly at the higher modulating frequencies. Although I have shown simple resistance and capacity elements for carrying out my invention it will be understood that I may use more complex arrangements of resistances, capacities, inductances, amplifiers, etc., in order to obtain varying increaseand decrease of response for various values otrate of change or equivalent modulating frequencies. I may, for example, shunt the capacity C of Figure 1 with a resistance so that the relative output modulation from modulator 40 holds fairly constant-up to certain modulating frequencies and then increases forhigher frequencies,

It may also be noted that my invention is ap-, plicableto photocell-amplifiers such as are 'used insound moving pictures in reproducing sound 1. In a system for enhancing the definition of indiciaj; reproduced by facsimile comprising I means for'scanning the indicia, means for producing current variations characteristic of the light jvai lles thereof, means forproducing potential variations characteristic of the rate of change ofzthe aforesaid current variations, an electron tube having an output electrode, and means for independently varying the impedance of the; tube in accordance with the produced currentand potential variations. v

I 2. In a system for producing controlling potentialsgcharacteristic of a wide band of signalling current comprising an indicia-bearlng surface, a source of light the intensityof which varies in accordance with the li ht values of said 4 aaoaeaa surface, an electron discharge device controlled in accordance with said variations in light intensity, said electron discharge device having its output electrode connected in an output circuit including an impedance, an additional electron discharge device having a first and a second control electrode and an output electrode, a utilizing circuit connected to said output electrode, a direct current circuit connecting the output electrode of said first named discharge device to said first control electrode, and a reactive circuit including a series resistance and condenser connecting the output electrode of said first-named device to said second co'ntrol electrode, whereby said second control electrode will be subjected to voltage variations corresponding to the rate of change of the voltage variations impressed on said first control electrode.

3. In a system for signalling, a source of light the intensity of which varies in accordance with indicia marking on a tape, an electron discharge device controlled in accordance with said variations in light intensity, said electron discharge device having its output electrode connected in an output circuit including an impedance, a second electron discharge device having a plurality of control electrodes and an output electrode, a utilization circuit connected to said output electrode, a circuit for directly connecting the output electrode of said first-named dis-v charge device to one of said control electrodes, and a circuit including a series resistance and reactance connecting the output electrode of said first named device to another of said control electrodes, whereby said second electron discharge device is controlled in accordance with the variations in light intensity and the rate of change of said variations.

4. In a facsimile transmitting system, the

method of controlling a thermionic tube having a plurality of control electrodes in accordance with messages comprising contrasting markings on a background which includes the steps of impressing on one of said control electrodes potential variations in accordance with the contrast ol the message, and-impressing on another oi said control electrodes potential variations 6 characteristic of the rate of change in contrast between various portions of said message.

5. The method of enhancing the definition ot a facsimile message comprising contrasting markings on a background which comprises the 10 steps of producing pulsating currents the intensity of which is characteristic of the contrast of said markings and background, producing a potential the amplitude of which varies in direct proportion to the pulsating currents, producing 15 a second potential the amplitude of which varies ,in accordance with the rate of change of the pulsating currents, and simultaneously utilizing both of the produced potentials to produce a single current variation. 7

6. The method of increasing the definition of facsimile messages comprising dark and light. contrasting markings which includes the steps of producing a potential variation characteristic of the contrast of the markings, producing a 25 second potential variation characteristic of the rate of change of the first produced potential variation, and individually utilizing the produced potentials to control a single electron stream.

"l. The method of increasing the variations of 30 signal modulations on a carrier wave characteristic of contrasting graphic markings which includes the steps of producing light variations characteristic of said contrasting markings, producing current variations in accordance with the .35

light variations, producing potential variations characteristic of the rate of change of said light I variations, combining the produced potential and current variations to produce a single signal series, and impressing the signal series on a car- 40 rier wave;

' W. HaNSELL. 

